1. Field of the Invention
The present invention relates to an apparatus and a method for forming an image with use of electrophotographic process, such as a digital copying machine, a digital printer or the like.
2. Description of Related Art
Various kinds of an apparatus such as a laser printer have been used practically for forming an image with use of electrophotographic process by exposing the photoconductor with a light beam according to image data which have been converted into digital values, while various kinds of methods for forming a digital image have been suggested in order to reproduce a half-tone image such as a picture with high fidelity.
Among such methods are known an area gradation method with use of a dither matrix a multi-level laser exposure method (such as the pulse width modulation method of the light-intensity modulation method) wherein the gradation for a dot to be printed is expressed by the amount of laser light (which is equal to the product of the light-emitting time with the light intensity) by changing the pulse width (light-emitting time) or the light intensity, (refer for example Japanese patents laid-open publication No. 91077/1987, No. 39972/1987, No. 188562/1987 and No. 22597/1986). Further, multi-level dither methods in combination of the dither method with the pulse width modulation method or light intensity modulation method are also known.
It is possible in principle to reproduce an image density with gradation corresponding uniquely to the degree of gradation of image data to be reproduced by using such a method. However, the image density to be reproduced (hereinafter referred to as image density) is not proportional to the density of a document owing to complex factors such as the photosensitive characteristic of the photoconductor, the characteristics of toners and the environment to be used. The nonlinear characteristic which deviates from the linear characteristic to be realized is called the gamma characteristic, generally, or the gradation characteristic, and this nonlinear gamma characteristic is one of the important factors to lower the fidelity of the reproduced image, especially of a half-tone document.
In order to improve the fidelity of the reproduced image, the so-called gamma correction is adopted, so that an image can be reproduced with fidelity according to the amplitude of the document image. In the gamma correction, a document density data to be read is transformed by using a predetermined transformation table for gamma correction, and a digital image is formed according to the transformed document density.
However, in the image reproduction with use of the multi-level laser exposure method or the multi-level dither method, a dot is expressed by an analog method or by using an electrostatic latent image on the photoconductor. Therefore, the gamma characteristic depends largely on the size of laser beam and the diffusion of the latent image. Thus, if the size of the laser beam is changed or the photosensitive characteristic of the photoconductor, especially the latent image diffusion rate, is changed, the gamma characteristic is also changed. The change of the size of laser beam may happen for example when a print head assembly having a laser diode as a principal part is replaced. The photosensitive characteristic of the photoconductor changed for example due to the scattering of the characteristic in the production process of the photoconductor or the deterioration of the photoconductor itself. Because the gamma correction with use of a single transformation table acts properly to result the linear gradation characteristic only if the gamma characteristic is constant, if the gamma characteristic changes, the gamma correction cannot be performed properly, and an image reproduced with fidelity can not be obtained. Further, the reproducibility of a narrow line is also affected by these problems especially in the light-intensity modulation method.